FOXP3, also known as scurfin, is a protein involved in immune system responses. A member of the forkhead box protein family, FOXP3 appears to function as a transcription factor in the development and function of regulatory T cells. In regulatory T cell model systems, the FOXP3 transcription factor occupies the promoters of many important for regulatory T-cell function, and may repress transcription of key genes following stimulation of T cell receptors.

Alterations in numbers of regulatory T-cells – in particular those that express FOXP3 – are found in a number of disease states. Patients with tumors have a local relative excess of FOXP3 positive T cells which inhibits the body’s ability to suppress the formation of cancerous cells.